The storage of electrochemical energy in molecular species is a critical challenge to many energy conversion strategies; from dye-sensitized solar cells to photo/electrocatalytic water splitting to redox flow batteries. Arguably, the best method to store electrochemical energy within molecules is by the formation of chemical bonds coupled to multi-electron oxidation/reduction reactions. For example, water splitting results in the 2e– reduction of 2H+ to H2 to form an H-H bond and the 4e– oxidation of 2H2O to O2 to form an O=O double bond.
Water oxidation and oxygen reduction are two fundamental reactions important for the storage and utilization and renewable energy resources. Water oxidation, often termed the oxygen evolution reaction (OER), represents the anodic half reaction responsible for conversion of electrical energy into chemical energy via water splitting. The other half reaction, hydrogen evolution (HER), generates H2 as a fuel source. The recombination of O2 and H2 inside a hydrogen fuel cell then converts this stored energy back to electricity when demanded. The oxygen reduction reaction (ORR) is therefore equally important to OER in terms of being able to effectively use the energy stored in H2 and O2.
Nanocrystalline CuMO2 (M = BIII, AlIII, GaIII, InIII, ScIII, CrIII) metal oxides are an attractive family of wide band gap p-type semiconductors with high demand for applications in organic photovoltaics, perovskite solar cells, and dye-sensitized solar cells. In these devices, CuMO2 materials act as hole transport layers whereby they facilitate the transfer/transport of electron vacancies (i.e. holes) from the photoactive layer to the external circuit. Currently, NiO is the most well studied p-type metal oxide within this field; however, serious challenges exist with regard to its low hole mobility and lack of transparency in the visible region.